1. Field of the Invention
This invention relates to an arrangement for cooling the rotor of an electric machine having a superconducting field winding and a co-rotating mixing chamber near the axis, which, during operation contains a phase mixture of a coolant supplied from the outside and from which liquid coolant for cooling the field winding is taken and to which the coolant is returned, discharge of gaseous coolant to the outside being provided for.
2. Description of the Prior Art
Such a cooling arrangement is described in the dissertation by A. Bejan: "Improved Thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator," Ph.D. Thesis, Massachusetts Institute of Technology (U.S.A.), December, 1974, page 151. In this cooling arrangement, a coolant, taken from an external coolant supply unit and expanded in a Joule-Thomson valve, is fed via a central coolant feed line to a co-rotating mixing chamber. As a result of Joule-Thomson expansion, a two-phase mixture of liquid and gaseous coolant is then contained in the mixing chamber. During rotating operation, the phases of this two-phase mixture are separated by the centrifugal forces acting on them, and the coolant vapor is collected in regions near the axis, and the coolant liquid, in regions away from the axis of the mixing chamber. A stream of liquid coolant is fed from the mixing chamber via radially disposed feed lines to the field winding, for instance, at an end face. The coolant then flows through the field winding in a direction parallel to the axis of rotation and is thereupon returned to the central mixing chamber via another radially disposed discharge line. The heat absorbed in the process causes the temperature of the coolant to rise and results in partial evaporation. The required pumping action for developing coolant flow through the field winding is brought about by a self-pumping effect based on density differences. The insentropically compressed coolant conducted radially outward in the feed lines is accelerated by the centrifugal forces and can thus pass into the field winding. Since it is warmed up by the dissipation occurring there or by heat transfer from the outside, its density decreases and a hydrostatic pressure difference occurs between the radial feed and return lines. Therefore, a pressure gradient develops along the winding between the inlet and the outlet point of the coolant, which leads to convection flow and causes the coolant to return, via the discharge lines, to the mixing chamber near the axis. (c.f., "Cryogenics," July 1977, pages 429 to 433, and German Offenlegenschrift No. 25 30 100).
In such a cooling arrangement, however, difficulties arise during the cooling-down phase of the field winding, since, during this phase, the rotor is not yet rotating or is rotating at low speed, so that there is practically no centrifugal force to urge the cold coolant into the still warm field winding. When the coolant enters the field winding, it is warmed up and evaporates and the gaseous coolant then forms a buffer which blocks the still colder coolant from flowing in. Additional steps must, therefore, be taken to ensure uniform cooling-down of the field winding during the cooling down phase. Usually, forced cooling is provided for this purpose. However, with this type of cooling, difficulties arise in later utilizing the above-mentioned self-pumping effect after the cooling-down phase has taken place.
It is therefore an object of the present invention to provide a cooling arrangement for the superconducting field winding of a rotor, in which these difficulties do not occur or are minimized. In particular, the cooling arrangement is to utilize the self-pumping effect when the apparatus is in operating condition, and it should also carry out cooling of the field winding during the cooling-down phase, in a way which is technically simple and effective from the point of view of heat exchange.